This invention relates to a scroll-type fluid machine. More particularly but not exclusively, it relates to a scroll-type vacuum pump of the type having two rotating scrolls.
A scroll-type fluid machine is a positive displacement machine having two interfitting elements usually referred to as scrolls. Each scroll includes a disk-shaped end plate and a thin-walled member, referred to as a spiral wrap, which projects perpendicularly from one surface of the end plate and curves outwards from the center of the end plate in the shape of an involute or other type of spiral. The two scrolls are disposed with the end plates parallel and the spiral wraps interfitting with one another so that they are in line contact at a plurality of locations. The surfaces of the end plates and the spiral wraps define a plurality of spiral compression chambers between the points of contact between the spiral wraps. If the scrolls are rotated with respect to one another in the proper direction while maintaining the line contact between the spiral wraps, the compression chambers are gradually moved towards the centers of the scrolls with an accompanying decrease in volume. A working fluid is introduced into the compression chambers via a suction port formed in the outer periphery of one of the scrolls and is then removed at a higher pressure from a discharge port formed in the center of the end plate of one of the scrolls.
In some scroll-type fluid machines which are employed as vacuum pumps, a check valve is disposed inside the discharge port to prevent gas and fluids from being sucked into the compression chamber from the outside of the pump through the discharge port. Such a check valve increases the degree of vacuum which can be produced by the pump. On the other hand, it has the disadvantage that the resistance of the check valve to fluid flow decreases the pumping efficiency of the pump.